The present invention concerns the field of nitrification of fluids, and particularly relates to the treatment of aqueous fluids and liquids to oxidize ammonia and nitrites to produce nitrates.
Ammonia and nitrite are toxic to higher life forms. Their concentration in effluent discharges and water for drinking is strictly controlled. It is known to use nitrifying bacteria to oxidize ammonia and nitrite to nitrate. This is a natural process in soil and water whereby ammonia is oxidized sequentially to the nitrite and thence to nitrate. Excess ammonia is often formed during the biological stage of conventional sewage treatment. It is also a by-product of various industries, for example the coking of coal, petroleum refining, certain chemical processes, animal husbandry and aquaculture.
In sewage treatment, a series of physical, chemical and biological processes are used to produce an effluent suitable for discharge into natural waterways. The biological treatment often results in the production of ammonia faster than it can be removed by nitrification, because the lithotrophic nitrifying bacteria have lower metabolic rates than the heterotrophic ammonia generators. Ammonia or nitrite content in the treated water returned to rivers or reservoirs is a toxic hazard. Hence where excess ammonia content is present in sewage treatment processes, a separate, dedicated nitrification process is used.
This involves the use of a trickling filter in which nitrifying bacteria are grown as a coherent layer on inert packing material layers disposed on very large trickling pans. Other prior art nitrification processes exist; these include activated sludge processes; rotating biological contactor processes, fixed or compacted bed treatment processes and fluidized bed processes. Attempts to improve the nitrification efficiency have resulted in the development of new processes in which nitrifying bacteria are immobilized on polystyrene beads and these beads are allowed to float on the waste water being treated. However, even these later processes are only capable of a nitrification rate of 1 kg N/m3xc2x7dxe2x88x921 (manufacturer""s figures), which is well below the normal rate of heterotrophic ammonia generation referred to above.
indeed, such microbial processes suffer from a number of drawbacks; in particular although these processes all make use of carriers with high surface areas, once the development of the microbial film reaches a certain level, the effective surface area is reduced significantly. Attempts to overcome this problem had been proposed whereby small particles, such as sand, have been used as carriers in bioreactors, but these frequently become clogged with suspended solids. In addition, the attached biofilms quickly bridge the particles thus again significantly reducing the effective surface area of the biofilm.
U.S. Pat. No 4,009,099 relates to a biological process for removing ammonia nitrogen from waste water by forming a fluidized bed of microorganisms attached to a solid particulate carrier continuously passing waste water to be treated through said fluidized bed, adding oxygen to said fluidized bed, retain the waste water in the fluidized bed for sufficient period of time while controlling other necessary parameters to biologically convert substantially all of the ammonia nitrogen to be removed from the waste water, to oxidize forms of nitrogen, water and cellular material and thence withdrawing the biologically converted products from the fluidized bed.
In operation of this process, however, as the ammonia oxidation reaction proceeds in the fluidized bed, bacteria grows on the surface of the carrier particles. After a time, if unchecked, the particles tend to agglomerate to form a layer which thickens to form a gelatinous mass. As a result the surface area per unit reactor volume available for biological reaction is greatly reduced; and the efficiency of the process effectively slumps. Furthermore, particles tend to be carried out in the fluid bed as their specific gravity decreases. They also tend to become attached to gas bubbles within the reactor which assists removal of the particles from the reactor.
In order to overcome these problems, excess bacterial growth is typically mechanically removed from the particles. The significant disadvantage of this process is the drawing of a fine balance between the vigour of the removal of the excess bacterial material on the one hand and the maintenance of a sufficient layer of biofilm on the particles on the other, to enable the process to continue.
European Patent Specification No 0861808 relates to a waste water treatment apparatus comprising a waste water treatment tank in which carrier particles support a surface layer of microbe are charged and decomposing and eliminating an organic matter and/or inorganic matter contained in a waste water, and a membrane module for filtering water to be treated which goes out from sand treatment tank, non-permeating water not passing through the membrane being retained and recirculated to the treatment tank. Such a system relies on a filtration system for further purifying water. In this process carriers for immobilizing microbes may comprise a vinyl alcohol resin and acrylic resin in combination with porous inorganic compounds. In this particular embodiment the combination of synthetic and inorganic carriers is required to constitute the carrier for the biomass.
U.S. Pat. No 5,747,311 relates to a method for chemical modifying a reaction medium using microbes, the method including providing particular material comprising a plastic carrier and microbes attached to the carrier in which the particulate material has a specific gravity, less than specific gravity of the dispersing fluid.
The disadvantage of the prior art methods is the problem of disposing of excess bacterial growth together with the potential loss of particulate material. The present invention seeks to overcome these disadvantages by providing a more stable support for the biomass.